Flexible NGL recovery methods and configurations

Abstract

A natural gas liquids plant uses a demethanizer and a deethanizer in a two-column or single column configuration that can be used for ethane recovery and ethane rejection. During ethane recovery, 95% ethane recovery and 99% propane recovery are achieved, while during ethane rejection the sales gas Wobbe Index requirement is maintained while maintaining 95% propane recovery. A residue gas recycle exchanger is most preferably configured to use the demethanizer overhead product to either cool a portion of the residue gas and a portion of the feed gas during ethane recovery, or to cool a portion of the feed gas using two distinct heat transfer areas to produce a feed gas reflux at significantly lower temperature.

What is claimed is: 
     
       1. A method of flexible ethane recovery in a gas processing plant for processing a feed gas including a first portion of the feed gas and a second portion of the feed gas, wherein the gas processing plant is configurable in either an ethane recovery configuration or an ethane rejection configuration, the method comprising:
 feeding a first reflux stream, a second reflux stream, and a demethanizer feed to a demethanizer; wherein the second portion of the feed gas forms the demethanizer feed after cooling and expansion, and wherein the first and second reflux streams are fed to the demethanizer at different first and second reflux locations; 
 routing a demethanizer overhead product stream to a residue gas recycle exchanger having a first and second heat transfer pathway through the recycle exchanger, wherein; 
 when the gas processing plant is in the ethane recovery configuration, the demethanizer overhead product stream is used to cool the first portion of the feed gas in the first heat transfer pathway to thereby produce the first reflux stream and to cool a portion of compressed residue gas in the second heat transfer pathway to thereby produce the second reflux stream; and 
 when the gas processing plant is in the ethane rejection configuration, the demethanizer overhead product is used to cool the first portion of the feed gas in each of the first and second heat transfer pathways to thereby produce the first and second reflux streams; and 
 using a demethanizer bottom to cool the second portion of the feed gas and feeding the demethanizer bottom after the cooling step to a deethanizer or deethanizer section of the demethanizer. 
 
     
     
       2. The method of  claim 1  further comprising using a plurality of switch valves to control switchover from the ethane rejection configuration to the ethane recovery configuration. 
     
     
       3. The method of  claim 1  wherein the second portion of the feed gas is cooled to −38° to −45° F. to partially condense the second portion of the feed gas, and further comprising separating the partially condensed second portion of the feed gas into a liquid fraction and a vapor fraction, and feeding the liquid and vapor fraction to the demethanizer at separate locations. 
     
     
       4. The method of  claim 3  further comprising a step of expanding the vapor fraction in a turbo expander and reducing pressure of the liquid fraction before feeding the liquid and vapor fraction to the demethanizer. 
     
     
       5. The method of  claim 1  further comprising a step of withdrawing an ethane stream as a deethanizer overhead product or deethanizer section overhead product. 
     
     
       6. The method of  claim 1  further comprising a step of compressing a deethanizer overhead product or deethanizer section overhead product and combining the compressed overhead product with the demethanizer overhead product during ethane rejection. 
     
     
       7. A gas processing plant for flexible ethane recovery when processing a feed gas including a first portion of the feed gas and a second portion of the feed gas, wherein the as processing plant is configurable in either an ethane recovery configuration or an ethane rejection configuration, wherein the gas processing plant comprises:
 a demethanizer configured to receive a first reflux stream and a second reflux stream, wherein the first reflux stream and the second reflux stream are received by the demethanizer at different first and second reflux locations; and 
 a residue gas recycle exchanger having a first and second heat transfer pathway through the recycle exchanger, the residue gas recycle exchanger being fluidly coupled to the demethanizer such that:
 when the gas processing plant is in the ethane recovery configuration, a demethanizer overhead product provides refrigeration to cool the first portion of the feed gas stream in the first heat transfer pathway to thereby produce the first reflux stream to the demethanizer and to cool a compressed residue gas stream in the second heat transfer pathway to thereby produce the second reflux stream to the demethanizer; and 
 when the gas processing plant is in the ethane rejection configuration, the demethanizer overhead product provides refrigeration to cool the portion of the feed gas in each of the first and second heat transfer pathways to thereby produce the first and second reflux streams. 
 
 
     
     
       8. The gas processing plant of  claim 7 , wherein the recycle exchanger is fluidly coupled to a plurality of switch valves that are configured to control switchover from the ethane rejection configuration to the ethane recovery configuration. 
     
     
       9. A gas processing plant for flexible ethane recovery, wherein the gas processing plant is configurable in either an ethane recovery configuration or an ethane rejection configuration, wherein the gas processing plant comprises:
 a feed gas source configured to provide a feed gas, including a first portion of the feed gas and a second portion of the feed gas; 
 a demethanizer configured to receive a demethanizer feed stream, a first reflux stream, and a second reflux stream, wherein the first reflux stream and the second reflux stream are received by the demethanizer at different first and second reflux locations, and wherein the demethanizer is also configured to produce a demethanizer overhead product and a demethanizer bottom product; 
 a deethanizer or deethanizer section fluidly coupled to the demethanizer such that the demethanizer bottom product is used to cool the second portion of the feed gas and is then fed to the deethanizer or deethanizer section, and wherein the deethanizer or deethanizer section is configured to produce a C3+ bottom product and a C2 enriched overhead product; 
 a residue gas recycle exchanger having a first and second heat transfer pathway through the recycle exchanger, the residue gas recycle exchanger being fluidly coupled to the demethanizer such that: 
 (a) when the gas processing plant is in the ethane recovery configuration, the demethanizer overhead product cools the first portion of the feed gas in the first heat transfer pathway to thereby produce the first reflux stream and cools a portion of compressed residue gas in the second heat transfer pathway to thereby produce the second reflux stream; and; 
 (b) when the gas processing plant in the ethane rejection configuration, the demethanizer overhead product cools the first portion of the feed gas in each of the first and second heat transfer pathways to thereby produce the first and second reflux streams. 
 
     
     
       10. The gas processing plant of  claim 9  further comprising using a plurality of switch valves that are configured to allow for switchover from the ethane rejection configuration, to the ethane recovery configuration. 
     
     
       11. The gas processing plant of  claim 9  further comprising a feed gas separator that is configured to receive a partially condensed second portion of the feed gas, to separate the partially condensed second portion of the feed gas into a liquid fraction and a vapor fraction, wherein the feed gas separator is fluidly coupled to the demethanizer to allow feeding the liquid and vapor fraction to the demethanizer at separate locations. 
     
     
       12. The gas processing plant of  claim 11  further comprising a turbo expander fluidly coupled between the feed gas separator and the demethanizer to expand the vapor fraction, and a JT valve fluidly coupled between the feed gas separator and the demethanizer to reduce pressure of the liquid fraction. 
     
     
       13. The gas processing plant of  claim 9  further comprising a conduit that is configured to allow withdrawal of the C2 enriched overhead product from the plant. 
     
     
       14. The gas processing plant of  claim 9  further comprising a compressor configured to compress the C2 enriched overhead product for combination with the demethanizer overhead product when the gas processing plant is configured in the ethane rejection configuration.